The Pax family of DNA-binding proteins includes essential regulators of tissue-specific gene expression in humans and other higher eukaryotes. Pax proteins are essential for the formation of differentiated cells and tissues, however, increasing levels of DNA binding activity results in neoplastic transformation and tumorigenesis. For example, chromosomal abnormalities and gene rearrangements resulting in overexpression of Pax-5 are associated with B lineage lymphomas in humans. Mechanisms contributing to Pax-5-mediated lymphomagenesis are not understood, but multiple lines of evidence suggest that the dosage of Pax-5 is exquisitely regulated in normal B cells by transcriptional and post-translational mechanisms. As one mechanism contributing to post-translational regulation of Pax-5, we propose that Pax- 5 DNA binding is regulated, in part, by the redox status of highly conserved cysteine residues in its paired DNA-binding domain. Thus, DNA binding by Pax-5 (and other Pax family members) may be reduced in response to oxidative stress. To date, this hypothesis has only been tested using limited in vitro model systems that do not adequately reflect the complexity of homeostatic mechanisms governing transcriptional activity in vivo. Moreover, it has not been determined whether S-thiolation (glutathionylation) of Pax-5 is an important mechanism for controlling its activity in vivo. In this application, we propose a genetic approach that bypasses previously encountered deficiencies associated with transfection assays and other in vitro experimental protocols. Our experiments will address a relatively unexplored area of molecular biology with profound implications for understanding how cells maintain precise levels of key regulatory factors. Our studies will eventually aid in devising new therapeutics for treating Pax-related cancers (lymphoma, astrocytoma, and rhabdomyosarcoma).